The present invention relates to a method for manufacturing a core, such as a rotor core, of a rotating electric machine, such as a motor.
FIG. 8 shows a known structure of a rotor core for a motor. In this structure, a rotor core body 31 includes receptacles 32 arranged along the peripheral portion of the rotor core body 31. Each receptacle 32 accommodates a permanent magnet 33. An anchoring material 34 is injected into the gap between the wall surface of each receptacle 32 and the outer surface of the corresponding permanent magnet 33 to anchor the permanent magnet 33 to the receptacle 32. Generally, an epoxy resin that is a thermosetting resin is used as the anchoring material 34.
In the prior art, the rotor core is manufactured through the steps shown in FIG. 9.
In a first step 35, core plates, which are pressed out of a metal plate, are stacked to form the rotor core body 31. In a second step 36, the permanent magnet 33 is inserted into and accommodated in each receptacle 32 of the rotor core body 31. In a third step 37, the rotor core body 31, which accommodates the permanent magnets 33, is heated to approximately 150° C. with a heating furnace or the like. Since an epoxy resin, which is a thermosetting resin, is used as the anchoring material 34 that anchors the accommodated permanent magnets 33, the rotor core body 31 and the permanent magnets 33 need to be preheated prior to the injection of the anchoring material 34.
When the rotor core body 31 and the permanent magnets 33 have been heated, in a fourth step 38, the anchoring material 34 is molten and injected into the gap between the wall surface of each receptacle 32 and the outer surface of the corresponding permanent magnet 33 using a molding tool. The anchoring material 34 is thermally cured to anchor each permanent magnet 33 in the corresponding receptacle 32. In a fifth step 39, the rotor core, which has the permanent magnets 33 accommodated and anchored in the receptacles 32, is cooled to room temperature by a cooling unit or the like.
The first to fifth steps manufacture the rotor core. Subsequently, the rotor core undergoes various steps such as the coupling of a rotor shaft to the rotor core.
Japanese Laid-Open Patent Publication Nos. 2013-136725 and 2013-183527 disclose the use of a thermosetting resin such as an epoxy resin as the anchoring material that fixes the magnets in the receptacles.
In Japanese Laid-Open Patent Publication No. 2010-141989, a thermosetting resin is not used. Each receptacle accommodates a plurality of permanent magnets. A resin foam sheet is arranged between the permanent magnets and expanded to press the permanent magnets against the wall surface of the corresponding receptacle. This anchors the permanent magnets to the rotor core body.
In Japanese Laid-Open Patent Publication No. 2005-192264, a thermosetting resin is not used. Instead, a thermoplastic resin, which is formed of a liquid crystal polymer (LCP), is molded to anchor permanent magnets to magnet embedment portions.
The general prior art and Japanese Laid-Open Patent Publication Nos. 2013-136725 and 2013-183527 described above use a thermosetting resin as the anchoring material. The thermosetting resin is injected into the receptacles of the rotor core body, which is heated to 150 degrees Celsius. Thus, a large amount of energy is used to heat the rotor core body, and the time used for heating and cooling lengthens the cycle time. Further, the thermal curing of the thermosetting resin is a chemical reaction. Thus, it takes time for the curing to be completed. Consequently, the use of a thermosetting resin as the anchoring material decreases the manufacturing efficiency of the rotor core.
The characteristics of the permanent magnets 33 are in that each permanent magnet 33 expands along an axis that is orthogonal to the axis of easy magnetization when the temperature decreases and contracts along the axis orthogonal to the axis of easy magnetization when the temperature increases. Thus, when the rotor core body 31 is heated, the rotor core body 31 expands while the permanent magnets 33 contract. This expands the gap between the wall surface of each receptacle 32 and the corresponding permanent magnet 33. The thermosetting resin is injected into the expanded gap. When the rotor core body 31 is cooled after the thermosetting resin is injected into the gap, the rotor core body 31 contracts and the permanent magnets 33 expand as shown by the arrows in FIG. 8. Thus, the cured thermosetting resin deforms the rotor core body 31. In particular, thin portions 311 of the rotor core body 31 may be greatly deformed toward the outer circumference of the rotor core body 31. This will vary the clearance between the rotor core body 31 and a stator, which is located at the outer side of the rotor core body 31, from the predetermined size and adversely affect the performance of the rotating electric machine.
Further, the epoxy resin that forms the anchoring material has poor ductility. Thus, cracks may form in the epoxy resin, or the anchoring material, when the epoxy resin is squeezed by the contraction of the rotor core body 31 and the expansion of the permanent magnets 33 and when the epoxy resin receives a heat shock or physical stress during use of the rotor core as a rotating electric machine. To prevent scattering of the cracked anchoring material, a cover may need to be arranged beside the rotor core body. This increases the size and weight of the rotor and lowers the operation efficiency of the rotating electric machine.
To shorten the time for injecting the thermosetting resin into the gaps, an injection nozzle of a molding tool may be increased in diameter. However, this increases the torque required to separate the molded product from the thermosetting resin in the nozzle after the thermosetting resin is cured. Thus, it becomes difficult to break away the thermosetting resin at the nozzle. In addition, the time required for the thermosetting resin to cure at the nozzle increases as the diameter of the injection nozzle increases.
This lengthens the cycle time for manufacturing the rotor core. This also increases the amount of thermosetting resin cull that is disposed of and increases the ratio of material loss.
Japanese Laid-Out Patent Publication No. 2010-141989 does not use a thermosetting resin. Thus, problems caused by the use of a thermosetting resin do not occur. However, in Japanese Laid-Out Patent Publication No. 2010-141989, the expansion pressure of the foam resin is used to anchor the permanent magnets. Thus, the permanent magnets are strongly pressed against the wall surfaces of the receptacles. The expansion amount of the foam resin, which is affected by the foams, is larger than the expansion amount of a thermosetting resin. Thus, the rotor core body deforms more easily than when using a thermosetting resin as the anchoring material. This may adversely affect the performance of the rotating electric machine.
Japanese Laid-Out Patent Publication No. 2005-192264 discloses the use of a thermoplastic resin, which is formed of LCP, instead of a thermosetting resin to anchor the permanent magnets. However, the conditions related to LCP are not disclosed.